CCNA Security
Chapter Seven
Cryptographic Systems
© 2009 Cisco Learning Institute.
1
Lesson Planning
• This lesson should take 3-4 hours to present
• The lesson should include lecture,
demonstrations, discussions and assessments
• The lesson can be taught in person or using
remote instruction
© 2009 Cisco Learning Institute.
2
Major Concepts
• Describe how the types of encryption, hashes,
and digital signatures work together to provide
confidentiality, integrity, and authentication
• Describe the mechanisms to ensure data
integrity and authentication
• Describe the mechanisms used to ensure data
confidentiality
• Describe the mechanisms used to ensure data
confidentiality and authentication using a public
key
© 2009 Cisco Learning Institute.
3
Lesson Objectives
Upon completion of this lesson, the successful participant
will be able to:
1. Describe the requirements of secure communications including
integrity, authentication, and confidentiality
2. Describe cryptography and provide an example
3. Describe cryptanalysis and provide an example
4. Describe the importance and functions of cryptographic hashes
5. Describe the features and functions of the MD5 algorithm and of
the SHA-1 algorithm
6. Explain how we can ensure authenticity using HMAC
7. Describe the components of key management
© 2009 Cisco Learning Institute.
4
Lesson Objectives
8. Describe how encryption algorithms provide confidentiality
9. Describe the function of the DES algorithms
10. Describe the function of the 3DES algorithm
11. Describe the function of the AES algorithm
12. Describe the function of the Software Encrypted Algorithm
(SEAL) and the Rivest ciphers (RC) algorithm
13. Describe the function of the DH algorithm and its supporting role
to DES, 3DES, and AES
14. Explain the differences and their intended applications
15. Explain the functionality of digital signatures
16. Describe the function of the RSA algorithm
17. Describe the principles behind a public key infrastructure (PKI)
© 2009 Cisco Learning Institute.
5
Lesson Objectives
18. Describe the various PKI standards
19. Describe the role of CAs and the digital certificates that they
issue in a PKI
20. Describe the characteristics of digital certificates and CAs
© 2009 Cisco Learning Institute.
6
Secure Communications
CSA
MARS
Firewall
VPN
IPS
CSA
VPN
Remote Branch
CSA
Iron Port
CSA
CSA
CSA
CSA
CSA
Web
Server
Email
Server
DNS
• Traffic between sites must be secure
• Measures must be taken to ensure it cannot be altered, forged, or
deciphered if intercepted
© 2009 Cisco Learning Institute.
7
Authentication
• An ATM Personal
Information Number (PIN)
is required for
authentication.
• The PIN is a shared
secret between a bank
account holder and the
financial institution.
© 2009 Cisco Learning Institute.
8
Integrity
• An unbroken wax seal on an envelop ensures integrity.
• The unique unbroken seal ensures no one has read the
contents.
© 2009 Cisco Learning Institute.
9
Confidentiality
I O D Q N H D V W
D W W D F N D W G D Z Q
© 2009 Cisco Learning Institute.
• Julius Caesar
would send
encrypted
messages to his
generals in the
battlefield.
• Even if
intercepted, his
enemies usually
could not read, let
alone decipher,
the messages.
10
History
Scytale - (700 BC)
Vigenère table
German Enigma Machine
Jefferson encryption device
© 2009 Cisco Learning Institute.
11
Transposition Ciphers
1
FLANK EAST
ATTACK AT DAWN
The clear text message would be
encoded using a key of 3.
Clear Text
2
F...K...T...T...A...W.
.L.N.E.S.A.T.A.K.T.A.N
..A...A...T...C...D...
Use a rail fence cipher and a
key of 3.
FKTTAW
LNESATAKTAN
AATCD
The clear text message would
appear as follows.
3
Ciphered Text
© 2009 Cisco Learning Institute.
12
Substitution Ciphers
Caesar Cipher
1
FLANK EAST
ATTACK AT DAWN
The clear text message would be
encoded using a key of 3.
Clear text
2
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
3
IODQN HDVW
DWWDFN DW GDZQ
Shift the top
scroll over by
three characters
(key of 3), an A
becomes D, B
becomes E, and
so on.
The clear text message would
be encrypted as follows using a
key of 3.
Cipherered text
© 2009 Cisco Learning Institute.
13
Cipher Wheel
1
FLANK EAST
ATTACK AT DAWN
The clear text message would be
encoded using a key of 3.
Clear text
2
Shifting the inner wheel by 3, then
the A becomes D, B becomes E,
and so on.
3
IODQN HDVW
DWWDFN DW GDZQ
The clear text message would
appear as follows using a key of 3.
Cipherered text
© 2009 Cisco Learning Institute.
14
Vigenѐre Table
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© 2009 Cisco Learning Institute.
15
Stream Ciphers
• Invented by the Norwegian Army Signal
Corps in 1950, the ETCRRM machine
uses the Vernam stream cipher method.
• It was used by the US and Russian
governments to exchange information.
• Plain text message is eXclusively OR'ed
with a key tape containing a random
stream of data of the same length to
generate the ciphertext.
• Once a message was enciphered the
key tape was destroyed.
• At the receiving end, the process was
reversed using an identical key tape to
decode the message.
© 2009 Cisco Learning Institute.
16
Defining Cryptanalysis
Allies decipher secret
NAZI encryption code!
Cryptanalysis is from the Greek words kryptós (hidden), and analýein
(to loosen or to untie). It is the practice and the study of determining
the meaning of encrypted information (cracking the code), without
access to the shared secret key.
© 2009 Cisco Learning Institute.
17
Cryptanalysis Methods
Brute Force Attack
Known Ciphertext
Successfully
Unencrypted
Key found
With a Brute Force attack, the attacker has some portion of
ciphertext. The attacker attempts to unencrypt the ciphertext with
all possible keys.
© 2009 Cisco Learning Institute.
18
Meet-in-the-Middle Attack
Known Ciphertext
Use every possible
decryption key until a result
is found matching the
corresponding plaintext.
Known Plaintext
Use every possible
encryption key until a
result is found matching
the corresponding
ciphertext.
MATCH of
Ciphertext!
Key found
With a Meet-in-the-Middle attack, the attacker has some portion of text
in both plaintext and ciphertext. The attacker attempts to unencrypt
the ciphertext with all possible keys while at the same time encrypt the
plaintext with another set of possible keys until one match is found.
© 2009 Cisco Learning Institute.
19
Choosing a Cryptanalysis Method
The graph outlines the
frequency of letters in the
English language.
1
For example, the letters E,
T and A are the most
popular.
There are 6 occurrences of the cipher
letter D and 4 occurrences of the cipher
letter W.
2
IODQN HDVW
DWWDFN DW GDZQ
Cipherered text
© 2009 Cisco Learning Institute.
Replace the cipher letter D first with
popular clear text letters including E, T,
and finally A.
Trying A would reveal the shift pattern of 3.
20
Defining Cryptology
Cryptology
+
Cryptography
© 2009 Cisco Learning Institute.
Cryptanalysis
21
Cryptanalysis
© 2009 Cisco Learning Institute.
22
Cryptographic Hashes, Protocols,
and Algorithm Examples
Integrity
MD5
SHA
Authentication
Confidentiality
HMAC-MD5
HMAC-SHA-1
RSA and DSA
DES
3DES
AES
SEAL
RC (RC2, RC4, RC5, and RC6)
HASH
NIST
© 2009 Cisco Learning Institute.
HASH w/Key
Rivest
Encryption
23
Hashing Basics
• Hashes are used for
integrity assurance.
Data of Arbitrary
Length
• Hashes are based on
one-way functions.
• The hash function hashes
arbitrary data into a fixedlength digest known as
the hash value, message
digest, digest, or
fingerprint.
© 2009 Cisco Learning Institute.
Fixed-Length
Hash Value
e883aa0b24c09f
24
Hashing Properties
Arbitrary
length text
X
Why is x not in
Parens?
h = H (x)
Hash
Function
Hash
Value
© 2009 Cisco Learning Institute.
(H)
Why is H in
Parens?
h
e883aa0b24c09f
25
Hashing in Action
• Vulnerable to man-in-the-middle attacks
- Hashing does not provide security to transmission.
• Well-known hash functions
I would like to
cash this
check.
- MD5 with 128-bit hashes
- SHA-1 with 160-bit hashes
Internet
Pay to Terry Smith
$100.00
Pay to Alex Jones
$1000.00
One Hundred and xx/100
Dollars
One Thousand and
xx/100 Dollars
4ehIDx67NMop9
12ehqPx67NMoX
Match = No changes
No match = Alterations
© 2009 Cisco Learning Institute.
26
MD5
• MD5 is a ubiquitous hashing
algorithm
• Hashing properties
- One-way function—easy to
compute hash and infeasible to
compute data given a hash
MD5
- Complex sequence of simple
binary operations (XORs,
rotations, etc.) which finally
produces a 128-bit hash.
© 2009 Cisco Learning Institute.
27
SHA
• SHA is similar in design to the MD4 and
MD5 family of hash functions
- Takes an input message of no more than 264 bits
- Produces a 160-bit message digest
• The algorithm is slightly slower than MD5.
SHA
• SHA-1 is a revision that corrected an
unpublished flaw in the original SHA.
• SHA-224, SHA-256, SHA-384, and SHA512 are newer and more secure versions of
SHA and are collectively known as SHA-2.
© 2009 Cisco Learning Institute.
28
Hashing Example
In this example the clear text entered is displaying hashed
results using MD5, SHA-1, and SHA256. Notice the
difference in key lengths between the various algorithm. The
longer the key, the more secure the hash function.
© 2009 Cisco Learning Institute.
29
Features of HMAC
• Uses an additional secret
key as input to the hash
function
Data of Arbitrary
Length
+
Secret
Key
• The secret key is known
to the sender and receiver
- Adds authentication to
integrity assurance
- Defeats man-in-the-middle
attacks
• Based on existing hash
functions, such as MD5
and SHA-1.
© 2009 Cisco Learning Institute.
Fixed Length
Authenticated
Hash Value
e883aa0b24c09f
The same procedure is used for
generation and verification of
secure fingerprints
30
HMAC Example
Data
Received Data
Pay to Terry Smith
$100.00
One Hundred and xx/100
Dollars
HMAC
(Authenticated
Fingerprint)
Secret
Key
4ehIDx67NMop9
Pay to Terry Smith
$100.00
One Hundred and xx/100
Dollars
4ehIDx67NMop9
© 2009 Cisco Learning Institute.
Pay to Terry Smith
$100.00
One Hundred and xx/100
Dollars
HMAC
(Authenticated
Fingerprint)
Secret Key
4ehIDx67NMop9
If the generated HMAC matches the
sent HMAC, then integrity and
authenticity have been verified.
If they don’t match, discard the
message.
31
Using Hashing
Data Authenticity
Data Integrity
e883aa0b24c09f
Fixed-Length Hash
Value
Entity Authentication
• Routers use hashing with secret keys
• Ipsec gateways and clients use hashing algorithms
• Software images downloaded from the website have checksums
• Sessions can be encrypted
© 2009 Cisco Learning Institute.
32
Key Management
Key Generation
Key Verification
Key
Management
Key Storage
Key Exchange
Key Revocation and Destruction
© 2009 Cisco Learning Institute.
33
Keyspace
DES Key
Keyspace
# of Possible Keys
256
56-bit
11111111 11111111 11111111
11111111 11111111 11111111 11111111
72,000,000,000,000,000
Twice as
much time
57
2
57-bit
11111111 11111111 11111111
11111111 11111111 11111111 11111111 1
144,000,000,000,000,000
Four time as
much time
258
58-bit
11111111 11111111 11111111
11111111 11111111 11111111 11111111 11
288,000,000,000,000,000
259
59-bit
11111111 11111111 11111111
11111111 11111111 11111111 11111111 111
60-bit
11111111 11111111 11111111
11111111 11111111 11111111 11111111 1111
576,000,000,000,000,000
With 60-bit DES
an attacker would
require sixteen
more time than
56-bit DES
260
1,152,000,000,000,000,000
For each bit added to the DES key, the attacker would require twice the amount of time to
search the keyspace.
Longer keys are more secure but are also more resource intensive and can affect throughput.
© 2009 Cisco Learning Institute.
34
Types of Keys
Symmetric
Key
Asymmetric
Key
Digital
Signature
Hash
80
1248
160
160
Protection up
to 10 years
96
1776
192
192
Protection up
to 20 years
112
2432
224
224
Protection up
to 30 years
128
3248
256
256
Protection against
quantum computers
256
15424
512
512
Protection up
to 3 years
 Calculations are based on the fact that computing power will continue to
grow at its present rate and the ability to perform brute-force attacks will
grow at the same rate.
 Note the comparatively short symmetric key lengths illustrating that
symmetric algorithms are the strongest type of algorithm.
© 2009 Cisco Learning Institute.
35
Key Properties
Shorter keys = faster
processing, but less secure
Longer keys = slower
processing, but more
secure
© 2009 Cisco Learning Institute.
36
Confidentiality and the OSI Model
• For Data Link Layer confidentiality, use proprietary linkencrypting devices
• For Network Layer confidentiality, use secure Network
Layer protocols such as the IPsec protocol suite
• For Session Layer confidentiality, use protocols such as
Secure Sockets Layer (SSL) or Transport Layer Security
(TLS)
• For Application Layer confidentiality, use secure e-mail,
secure database sessions (Oracle SQL*net), and secure
messaging (Lotus Notes sessions)
© 2009 Cisco Learning Institute.
37
Symmetric Encryption
Key
Encrypt
$1000
Pre-shared
key
$!@#IQ
Key
Decrypt
$1000
• Best known as shared-secret key algorithms
• The usual key length is 80 - 256 bits
• A sender and receiver must share a secret key
• Faster processing because they use simple mathematical operations.
• Examples include DES, 3DES, AES, IDEA, RC2/4/5/6, and Blowfish.
© 2009 Cisco Learning Institute.
38
Symmetric Encryption and XOR
The XOR operator results in a 1 when the value of
either the first bit or the second bit is a 1
The XOR operator results in a 0 when neither or both
of the bits is 1
Plain Text
1
1
0
1
0
0
1
1
Key (Apply)
0
1
0
1
0
1
0
1
XOR (Cipher Text)
1
0
0
0
0
1
1
0
Key (Re-Apply)
0
1
0
1
0
1
0
1
XOR (Plain Text)
1
1
0
1
0
0
1
1
© 2009 Cisco Learning Institute.
39
Asymmetric Encryption
Encryption Key
Encrypt
$1000
Two separate
keys which are
not shared
%3f7&4
Decryption Key
Decrypt
$1000
• Also known as public key algorithms
• The usual key length is 512–4096 bits
• A sender and receiver do not share a secret key
• Relatively slow because they are based on difficult computational
algorithms
• Examples include RSA, ElGamal, elliptic curves, and DH.
© 2009 Cisco Learning Institute.
40
Asymmetric Example : Diffie-Hellman
Get Out Your Calculators?
© 2009 Cisco Learning Institute.
41
Symmetric Algorithms
Symmetric
Encryption
Algorithm
DES
3DES
AES
Key length
(in bits)
56
Description
Designed at IBM during the 1970s and was the NIST standard until 1997.
Although considered outdated, DES remains widely in use.
Designed to be implemented only in hardware, and is therefore extremely
slow in software.
112 and 168
Based on using DES three times which means that the input data is
encrypted three times and therefore considered much stronger than DES.
However, it is rather slow compared to some new block ciphers such as AES.
128, 192, and 256
Fast in both software and hardware, is relatively easy to implement, and
requires little memory.
As a new encryption standard, it is currently being deployed on a large scale.
Software
Encryption
Algorithm (SEAL)
160
SEAL is an alternative algorithm to DES, 3DES, and AES.
It uses a 160-bit encryption key and has a lower impact to the CPU when
compared to other software-based algorithms.
The RC series
RC2 (40 and 64)
RC4 (1 to 256)
RC5 (0 to 2040)
RC6 (128, 192,
and 256)
A set of symmetric-key encryption algorithms invented by Ron Rivest.
RC1 was never published and RC3 was broken before ever being used.
RC4 is the world's most widely used stream cipher.
RC6, a 128-bit block cipher based heavily on RC5, was an AES finalist
developed in 1997.
© 2009 Cisco Learning Institute.
42
Symmetric Encryption Techniques
blank blank 1100101 01010010110010101
64 bits
64bits
01010010110010101
64bits
Block Cipher – encryption is completed
in 64 bit blocks
0101010010101010100001001001001 0101010010101010100001001001001
Stream Cipher – encryption is one bit
at a time
© 2009 Cisco Learning Institute.
43
Selecting an Algorithm
DES
3DES
AES
The algorithm is trusted by
the cryptographic
community
Been
replaced by
3DES
Yes
Verdict is
still out
The algorithm adequately
protects against brute-force
attacks
No
Yes
Yes
© 2009 Cisco Learning Institute.
44
DES Scorecard
Description
Timeline
Data Encryption Standard
Standardized 1976
Type of Algorithm
Symmetric
Key size (in bits)
56 bits
Speed
Time to crack
(Assuming a computer could try
255 keys per second)
Resource
Consumption
© 2009 Cisco Learning Institute.
Medium
Days (6.4 days by the COPACABANA machine, a specialized
cracking device)
Medium
45
Block Cipher Modes
ECB
CBC
Message of Five 64-Bit Blocks
Message of Five 64-Bit Blocks
Initialization
Vector
DES
DES
DES
DES
DES
DES
DES
DES
DES
DES
© 2009 Cisco Learning Institute.
46
Considerations
• Change keys frequently to help
prevent brute-force attacks.
DES
• Use a secure channel to
communicate the DES key from
the sender to the receiver.
• Consider using DES in CBC
mode. With CBC, the
encryption of each 64-bit block
depends on previous blocks.
• Test a key to see if it is a weak
key before using it.
© 2009 Cisco Learning Institute.
47
3DES Scorecard
Description
Timeline
Triple Data Encryption Standard
Standardized 1977
Type of Algorithm
Symmetric
Key size (in bits)
112 and 168 bits
Speed
Time to crack
(Assuming a computer could try
255 keys per second)
Resource
Consumption
© 2009 Cisco Learning Institute.
Low
4.6 Billion years with current technology
Medium
48
Encryption Steps
1
2
© 2009 Cisco Learning Institute.
The clear text from Alice is
encrypted using Key 1. That
ciphertext is decrypted
using a different key, Key 2.
Finally that ciphertext is
encrypted using another
key, Key 3.
When the 3DES ciphered text
is received, the process is
reversed. That is, the
ciphered text must first be
decrypted using Key 3,
encrypted using Key 2, and
finally decrypted using Key 1.
49
AES Scorecard
Description
Timeline
Advanced Encryption Standard
Official Standard since 2001
Type of Algorithm
Symmetric
Key size (in bits)
128, 192, and 256
Speed
Time to crack
(Assuming a computer could try
255 keys per second)
Resource
Consumption
© 2009 Cisco Learning Institute.
High
149 Trillion years
Low
50
Advantages of AES
• The key is much stronger due to the key length
• AES runs faster than 3DES on comparable hardware
• AES is more efficient than DES and 3DES on
comparable hardware
The plain text is now
encrypted using 128
AES
An attempt at
deciphering the text
using a lowercase,
and incorrect key
© 2009 Cisco Learning Institute.
51
SEAL Scorecard
Description
Timeline
Software-Optimized Encryption Algorithm
First published in 1994. Current version is 3.0 (1997)
Type of Algorithm
Symmetric
Key size (in bits)
160
Speed
High
Time to crack
(Assuming a computer could try
255 keys per second)
Resource
Consumption
© 2009 Cisco Learning Institute.
Unknown but considered very safe
Low
52
Rivest Codes Scorecard
Description
RC2
RC4
RC5
RC6
Timeline
1987
1987
1994
1998
Type of Algorithm
Block cipher
Stream
cipher
Block cipher Block cipher
1 - 256
0 to 2040
bits (128
suggested)
Key size (in bits)
© 2009 Cisco Learning Institute.
40 and 64
128, 192, or
256
53
DH Scorecard
Description
Timeline
Diffie-Hellman Algorithm
1976
Type of Algorithm Asymmetric
Key size (in bits)
Speed
Time to crack
(Assuming a computer could
try 255 keys per second)
Resource
Consumption
© 2009 Cisco Learning Institute.
512, 1024, 2048
Slow
Unknown but considered very safe
Medium
54
Using Diffie-Hellman
Alice
Shared
1
Bob
Calc
Secret
Shared
5, 23
1
Secret
Calc
5, 23
3
2
6
56mod 23 =
8 8
1. Alice and Bob agree to use the same two numbers. For example, the base number
g=
5 and prime number p=23
2. Alice now chooses a secret number x=
6.
56 modulo 23) = 8 (Y) and
3. Alice performs the DH algorithm: gx modulo p = (
sends the new number
© 2009 Cisco Learning Institute.
8 (Y) to Bob.
55
Using Diffie-Hellman
Alice
Shared
Bob
Calc
Secret
Shared
Calc
Secret
5, 23
5, 23
6
8 8
19
19 mod 23 = 2
56mod 23 =
5
6
15
4
515mod 23 = 19
6
815mod 23 =
15, performed the DH algorithm:
modulo p = (515 modulo 23) = 19 (Y) and sent the new number 19 (Y) to
4. Meanwhile Bob has also chosen a secret number x=
gx
Alice.
196 modulo 23) = 2.
modulo p = (86 modulo 23) = 2.
5. Alice now computes Yx modulo p = (
6. Bob now computes Yx
© 2009 Cisco Learning Institute.
The result (2) is the same
for both Alice and Bob.
This number can now be
used as a shared secret
key by the encryption
algorithm.
56
2
Asymmetric Key Characteristics
Encryption
Key
Encryption
Plain
text
Encrypted
text
Decryption
Key
Decryption
Plain
text
• Key length ranges from 512–4096 bits
• Key lengths greater than or equal to 1024 bits can be
trusted
• Key lengths that are shorter than 1024 bits are
considered unreliable for most algorithms
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57
Public Key (Encrypt) + Private Key
(Decrypt) = Confidentiality
Computer A acquires
Computer B’s public key
Bob’s Public
Key
Can I get your Public Key please?
1
Here is my Public Key.
2
Bob’s Public
Key
Computer
A
Computer A transmits
The encrypted message
to Computer B
Computer
B
Encrypted
Text
Encryption
Encryption
Algorithm
Algorithm
Encrypted
Text
3
Computer A uses Computer B’s
public key to encrypt a message
using an agreed-upon algorithm
© 2009 Cisco Learning Institute.
Bob’s Private
Key
4
Computer B uses
its private key to
decrypt and reveal
the message
58
Private Key (Encrypt) + Public Key
(Decrypt) = Authentication
Alice encrypts a message
with her private key
1
Alice’s Private
Key
Encrypted
Text
Encryption
Algorithm
2
Computer
A
Bob uses the public key to
successfully decrypt the message
and authenticate that the message
did, indeed, come from Alice.
Alice transmits the
encrypted message
to Bob
Encrypted
Text
Alice’s Public
Key
3
Can I get your Public Key please?
4
Alice’s Public
Key
Encrypted
Text
Computer
B
Encryption
Algorithm
Here is my Public Key
Bob needs to verify that the message
actually came from Alice. He requests
and acquires Alice’s public key
© 2009 Cisco Learning Institute.
59
Asymmetric Key Algorithms
DH
Digital Signature
Standard (DSS) and
Digital Signature
Algorithm (DSA)
RSA encryption
algorithms
EIGamal
Elliptical curve
techniques
© 2009 Cisco Learning Institute.
Key
length
(in bits)
Description
512, 1024,
2048
Invented in 1976 by Whitfield Diffie and Martin Hellman.
Two parties to agree on a key that they can use to encrypt messages
The assumption is that it is easy to raise a number to a certain power, but difficult
to compute which power was used given the number and the outcome.
512 - 1024
Created by NIST and specifies DSA as the algorithm for digital signatures.
A public key algorithm based on the ElGamal signature scheme.
Signature creation speed is similar with RSA, but is slower for verification.
512 to 2048
Developed by Ron Rivest, Adi Shamir, and Leonard Adleman at MIT in 1977
Based on the current difficulty of factoring very large numbers
Suitable for signing as well as encryption
Widely used in electronic commerce protocols
512 - 1024
Based on the Diffie-Hellman key agreement.
Described by Taher Elgamal in 1984and is used in GNU Privacy Guard software,
PGP, and other cryptosystems.
The encrypted message becomes about twice the size of the original message
and for this reason it is only used for small messages such as secret keys
160
Invented by Neil Koblitz in 1987 and by Victor Miller in 1986.
Can be used to adapt many cryptographic algorithms
Keys can be much smaller
60
Security Services- Digital Signatures
• Authenticates a source,
proving a certain party
has seen, and has signed,
the data in question
• Signing party cannot
repudiate that it signed
the data
• Guarantees that the data
has not changed from the
time it was signed
© 2009 Cisco Learning Institute.
Authenticity
Integrity
Nonrepudiation
61
Digital Signatures
• The signature is authentic and
not forgeable: The signature is
proof that the signer, and no one
else, signed the document.
• The signature is not reusable:
The signature is a part of the document and cannot be moved to a
different document.
• The signature is unalterable: After a document is signed, it cannot
be altered.
• The signature cannot be repudiated: For legal purposes, the
signature and the document are considered to be physical things. The
signer cannot claim later that they did not sign it.
© 2009 Cisco Learning Institute.
62
The Digital Signature Process
The sending device creates
a hash of the document
Data
Confirm
Order
The receiving device
accepts the document
with digital signature
and obtains the public key
Validity of the digital
signature is verified
Signature Verified
0a77b3440…
1
hash
Signature
Key
2
Encrypted
hash
Signed Data
Confirm
Order
____________
0a77b3440…
6
4
3
The sending device
encrypts only the hash
0a77b3440…
with the private key
of the signer
The signature algorithm
generates a digital signature
and obtains the public key
© 2009 Cisco Learning Institute.
Signature
Algorithm
Verification
Key
Signature is
verified with
the verification
key
5
63
Code Signing with Digital Signatures
• The publisher of the software attaches a digital signature to the
executable, signed with the signature key of the publisher.
• The user of the software needs to obtain the public key of the
publisher or the CA certificate of the publisher if PKI is used.
© 2009 Cisco Learning Institute.
64
DSA Scorecard
Description
Timeline
Digital Signature Algorithm (DSA)
1994
Type of Algorithm Provides digital signatures
Advantages:
Signature generation is fast
Disadvantages:
Signature verification is slow
© 2009 Cisco Learning Institute.
65
RSA Scorecard
Description
Timeline
Ron Rivest, Adi Shamir, and Len Adleman
1977
Type of Algorithm Asymmetric algorithm
Key size (in bits)
512 - 2048
Advantages:
Signature verification is fast
Disadvantages:
Signature generation is slow
© 2009 Cisco Learning Institute.
66
Properties of RSA
• One hundred times slower than
DES in hardware
• One thousand times slower
than DES in software
• Used to protect small amounts
of data
• Ensures confidentiality of data
thru encryption
• Generates digital signatures for
authentication and
nonrepudiation of data
© 2009 Cisco Learning Institute.
67
Public Key Infrastructure
Alice applies for a driver’s license.
She receives her driver’s license
after her identity is proven.
Alice attempts to cash a check.
Her identity is accepted after her
driver’s license is checked.
© 2009 Cisco Learning Institute.
68
Public Key Infrastructure
PKI terminology to remember:
PKI:
A service framework (hardware, software, people,
policies and procedures) needed to support largescale public key-based technologies.
Certificate:
A document, which binds together the name of the
entity and its public key and has been signed by the
CA
Certificate authority (CA):
The trusted third party that signs the public keys
of entities in a PKI-based system
© 2009 Cisco Learning Institute.
69
CA Vendors and Sample Certificates
http://www.verisign.com
http://www.entrust.com
http://www.verizonbusiness.com/
http://www.novell.com
http://www.rsa.com/
http://www.microsoft.com
© 2009 Cisco Learning Institute.
70
Usage Keys
• When an encryption certificate is used much more frequently than a
signing certificate, the public and private key pair is more exposed
due to its frequent usage. In this case, it might be a good idea to
shorten the lifetime of the key pair and change it more often, while
having a separate signing private and public key pair with a longer
lifetime.
• When different levels of encryption and digital signing are required
because of legal, export, or performance issues, usage keys allow an
administrator to assign different key lengths to the two pairs.
• When key recovery is desired, such as when a copy of a user’s
private key is kept in a central repository for various backup reasons,
usage keys allow the user to back up only the private key of the
encrypting pair. The signing private key remains with the user,
enabling true nonrepudiation.
© 2009 Cisco Learning Institute.
71
The Current State
X.509
• Many vendors have proposed and implemented
proprietary solutions
• Progression towards publishing a common set of
standards for PKI protocols and data formats
© 2009 Cisco Learning Institute.
72
X.509v3
• X.509v3 is a standard that
describes the certificate
structure.
• X.509v3 is used with:
- Secure web servers: SSL
and TLS
- Web browsers: SSL and
TLS
- Email programs: S/MIME
- IPsec VPNs: IKE
© 2009 Cisco Learning Institute.
73
X.509v3 Applications
SSL
External
Web Server
Enterprise
Network
Internet
Internet
Mail
Server
S/MIME
EAP-TLS
Cisco
Secure
ACS
CA
Server
IPsec
VPN
Concentrator
• Certificates can be used for various purposes.
• One CA server can be used for all types of authentication
as long as they support the same PKI procedures.
© 2009 Cisco Learning Institute.
74
RSA PKCS Standards
•
•
•
•
•
•
•
•
•
•
PKCS
PKCS
PKCS
PKCS
PKCS
PKCS
PKCS
PKCS
PKCS
PKCS
© 2009 Cisco Learning Institute.
#1: RSA Cryptography Standard
#3: DH Key Agreement Standard
#5: Password-Based Cryptography Standard
#6: Extended-Certificate Syntax Standard
#7: Cryptographic Message Syntax Standard
#8: Private-Key Information Syntax Standard
#10: Certification Request Syntax Standard
#12: Personal Information Exchange Syntax Standard
#13: Elliptic Curve Cryptography Standard
#15: Cryptographic Token Information Format Standard
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Public Key Technology
PKCS#7
PKCS#10
CA
Certificate
Signed
Certificate
PKCS#7
• A PKI communication protocol used for VPN PKI
enrollment
• Uses the PKCS #7 and PKCS #10 standards
© 2009 Cisco Learning Institute.
76
Single-Root PKI Topology
• Certificates issued by one CA
• Centralized trust decisions
• Single point of failure
Root CA
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77
Hierarchical CA Topology
Root CA
Subordinate
CA
• Delegation and distribution of trust
• Certification paths
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78
Cross-Certified CAs
CA2
CA1
CA3
• Mutual cross-signing of CA certificates
© 2009 Cisco Learning Institute.
79
Registration Authorities
CA
2
Completed Enrollment
Request Forwarded to
CA
Hosts will submit
certificate requests
to the RA
Enrollment
request
RA
After the Registration
Authority adds specific
information to the
certificate request and
the request is approved
under the organization’s
policy, it is forwarded
on to the Certification
Authority
3
1
Certificate Issued
The CA will sign the certificate
request and send it back to the
host
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80
Retrieving the CA Certificates
Alice and Bob telephone the CA
administrator and verify the public key
and serial number of the certificate
Out-of-Band
Authentication of
the CA Certificate
Out-of-Band
Authentication of
the CA Certificate
3
CA
Admin
POTS
3
POTS
CA
1
1
CA
Certificate
CA
Certificate
Enterprise Network
2
Alice and Bob request the CA certificate
that contains the CA public key
© 2009 Cisco Learning Institute.
2
Each system verifies the
validity of the certificate
81
Submitting Certificate Requests
The certificate is
retrieved and the
certificate is installed
onto the system
2
Out-of-Band
Authentication of
the CA Certificate
The CA administrator telephones to
confirm their submittal and the public
key and issues the certificate by
adding some additional data to the
request, and digitally signing it all
Out-of-Band
Authentication of
the CA Certificate
CA
Admin
POTS
POTS
CA
1
3
1
Certificate
Request
Certificate
Request
3
Enterprise Network
Both systems forward a certificate request which
includes their public key. All of this information is
encrypted using the public key of the CA
© 2009 Cisco Learning Institute.
82
Authenticating
Bob and Alice exchange certificates. The CA is no longer involved
2
2
Private Key (Alice)
Private Key (Bob)
Certificate (Alice)
1
Certificate (Alice)
Certificate (Bob)
Certificate (Bob)
CA Certificate
CA Certificate
Each party verifies the digital signature on the certificate by hashing the
plaintext portion of the certificate, decrypting the digital signature using the
CA public key, and comparing the results.
© 2009 Cisco Learning Institute.
83
PKI Authentication Characteristics
• To authenticate each other, users have to obtain
the certificate of the CA and their own certificate.
These steps require the out-of-band verification
of the processes.
• Public-key systems use asymmetric keys where
one is public and the other one is private.
• Key management is simplified because two
users can freely exchange the certificates. The
validity of the received certificates is verified
using the public key of the CA, which the users
have in their possession.
• Because of the strength of the algorithms,
administrators can set a very long lifetime for the
certificates.
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© 2009 Cisco Learning Institute.
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